The hypothalamic regulation of pituitary gonadotropin secretion and of sexual behavior in the male rat differ from that in the female, and these functional differences are established postnatally under the influence of testicular androgen. This research program is directed at identifying the neural basis of these sex differences in the adult and in elucidating their development. Recently we have discovered a marked morphological difference within the hypothalamus, and we propose to describe the ontogeny of this anatomical difference, to analyze quantitatively the cell types which produce the dimorphism, and to determine the "birthdates" of these cells and their possible function. In addition the electrophysiological approach of recording the activity of single neurons will be utilized to define developmental changes in hypothalamic connectivity, particularly within the sexually dimorphic region. Finally, we will continue to attempt to identify the neural substrate important for the regulation of gonadotropin or sexual behavior in the adult. We will focus on extrahypothalamic structures and on the possible neuroanatomical dissociation of these two fundamental aspects of reproduction. This broad approach is expected to provide further insight into the process of the sexual differentiation of the brain.